1. Field of the Invention
The present invention relates to RF plasma processing reactors and, more particularly, to an inventive plasma reactor which uses a radio frequency (RF) energy source and a multiple coil antenna for inductively coupling the associated RF electromagnetic wave to the plasma.
2. Description of the Related Technology
The trend toward increasingly dense integrated geometries has resulted in components and devices of very small geometry which are electrically sensitive and susceptible to damage when subjected to wafer sheath voltages as small as approximately 200-300 volts due to energetic particle bombardment or radiation. Unfortunately, such voltages are of smaller magnitude than the voltages to which the circuit components are subjected during standard integrated circuit fabrication processes.
Structures such as MOS capacitors and transistors fabricated for advanced devices have very thin (thickness  less than 200 Angstroms) gate oxides. These devices may be damaged by charge-up, resulting in gate breakdown. This can occur in a plasma process when neutralization of surface charge does not occur, by non-uniform plasma potential/or density, or by large RF displacement currents. Conductors such as interconnect lines may be damaged for similar reasons as well.
RF Systems
Consider first prior art semiconductor processing systems such as CVD (chemical vapor deposition) and RIE (reactive ion etching) reactor systems. These systems may use radio frequency energy at low frequencies from about 10-500 KHz up to higher frequencies of about 13.56-40.68 MHz. Below about 1 MHz, ions and electrons can be accelerated by the oscillating electric field, and by any steady state electric field developed in the plasma. At such relatively low frequencies, the electrode sheath voltage produced at the wafers typically is up to one or more kilovolts peak, which is much higher than the damage threshold of 200-300 volts. Above several MHz, electrons are still able to follow the changing electric field. More massive ions are not able to follow the changing field, but are accelerated by steady state electric fields. In this frequency range (and at practical gas pressures and power levels), steady state sheath voltages are in the range of several hundred volts to 1,000 volts or more.
Magnetic Field-Enhancement
A favorite approach for decreasing the bias voltage in RF systems involves applying a magnetic field to the plasma. This B field confines the electrons to the region near the surface of the wafer and increases the ion flux density and ion current and, thus, reduces the voltage and ion energy requirements. By way of comparison, an exemplary non-magnetic RIE process for etching silicon dioxide might use RF energy applied at 13.56 MHz, an asymmetrical system of 10-15 liters volume, 50 millitorr pressure and an anode area to wafer-support cathode area ratio of approximately (8-10) to 1, and develop wafer (cathode)sheath voltage of approximately 800 volts. The application of a magnetic field of 60 gauss may decrease the bias voltage approximately 25-30 percent, from 800 volts to about 500-600 volts, while increasing the etch rate by as much as about 50 percent.
However, the application of a stationary B field parallel to the wafer develops an Exc3x97B ion/electron drift and an associated plasma density gradient which is directed diametrically across the wafer. The plasma gradient causes non-uniform etching, deposition and other film properties across the wafer. The non-uniformities may be decreased by rotating the magnetic field around the wafer, typically either by mechanical movement of permanent magnets, or by using pairs of electromagnetic coils which are driven in quadrature, 90 degrees out of phase, or by instantaneously controlling the current in pairs of coils to step or otherwise move the magnetic field at a controlled rate. However, although rotating the field reduces the non-uniformity gradient, typically some degree of non-uniformity remains.
Furthermore, it is difficult to pack coils and, in particular, to pack two or more pairs of coils about a chamber and to achieve a compact system, especially when using a Helmholtz coil configuration and/or a multi-chamber system of individual magnetic-enhanced reactor chambers surrounding a common loadlock.
A unique reactor system which has the capability to instantaneously and selectively alter the magnetic field strength and direction, and which is designed for use in compact multi-chamber reactor systems, is disclosed in commonly assigned U.S. Pat. No. 4,842,683, issued Jun. 27, 1989, in the name of inventors Cheng et al.
Microwave/ECR Systems
Microwave and microwave ECR (electroncylotron resonance) systems use microwave energy of frequencies  greater than 800 MHz and, typically, frequencies of 2.45 GHz to excite the plasma. This technique produces a high density plasma, but low particle energies which may be below the minimum reaction threshold energy for many processes, such as the reactive ion etching of silicon dioxide. To compensate, energy-enhancing low frequency electrical power is coupled to the wafer support electrode and through the wafer to the plasma. Thus, the probability of wafer damage is decreased relative to previous systems.
Microwave and microwave ECR systems operated at practical power levels for semiconductor wafer processing such as etch or CVD require large waveguide for power transmission, and expensive tuners, directional couplers, circulators, and dummy loads for operation. Additionally, to satisfy the ECR condition for microwave ECR systems operated at the commercially available 2.45 GHz, a magnetic field of 875 gauss is necessitated, requiring large electromagnets, large power and cooling requirements.
Microwave and microwave ECR systems are not readily scalable. Hardware is available for 2.45 GHz, because this frequency is used for microwave ovens. 915 MHz systems are also available, although at higher cost. Hardware is not readily or economically available for other frequencies. As a consequence, to scale a 5-6 in. microwave system upward to accommodate larger semiconductor wafers requires the use of higher modes of operation. This scaling at a fixed frequency by operating at higher modes requires very tight process control to avoid so-called mode flipping to higher or lower order loads and resulting process changes. Alternatively, scaling can be accomplished, for example, for a 5-6 in. microwave cavity, by using a diverging magnetic field to spread out the plasma flux over a larger area. This method reduces effective power density and thus plasma density.
HF Transmission Line System
Previously mentioned, commonly assigned parent patent application U.S. Ser. No. 559,947, entitled VHF/UHF REACTOR SYSTEM, filed Jul. 31, 1990, in the name of the inventors Collins et al (AMAT File 151-1) is incorporated by reference. This incorporated application discloses a high frequency VHF/UHF reactor system in which the reactor chamber itself is configured in part as a transmission line structure for applying high frequency plasma generating energy to the chamber from a matching a network. The unique integral transmission line structure permits satisfaction of the requirements of a very short transmission line between the matching network and the load and permits the use of relatively high frequencies, 50 to 800 MHz. It enables the efficient, controllable application of RF plasma generating energy to the plasma electrodes for generating commercially acceptable etch and deposition rates at relatively low ion energies and low sheath voltages. The relatively low voltages reduce the probability of damage to electrically sensitive small geometry semiconductor devices. The VHF/UHF system avoids various other prior art shortcomings, such as the above-described scalability and power limitations.
In one aspect, our invention which overcomes prior art shortcomings is embodied in the construction and operation of an RF plasma processing system comprising a vacuum chamber having a source region and a processing region; means for inductively coupling RF electromagnetic energy into the processing chamber for generating a plasma within the chamber to fabricate an article such as a semiconductor wafer positioned, for example, at the coupling means or downstream relative to the coupling means; and a triode arrangement comprising an RF cathode in the processing region, an anode defined by the chamber walls, and a source region electrode which is electrically floating, grounded or connected to RF bias, for enhancing plasma processing. The construction of the source region electrode and/or the chamber walls defining the source region may include silicon for enhancing processes such as oxide etching.
Preferably, LF/VHF (low frequency to very high frequency) RF power within the range 100 KHz to 100 MHz is used. More preferably, LF/HF power within the range 100 KHz to 10 MHz is used. Most preferably, MF (medium frequency) power is used within the range 300 KHz to 3 MHz. Preferably, the coupling means is a multiple turn, cylindrical coil antenna of uncoiled electrical length  less than xcex/4 where xcex is the wavelength of the high frequency RF excitation energy applied to the coil antenna during plasma operation.
Our invention also encompasses means connected to the antenna for tuning the antenna to resonance, as well as load means connected to the antenna to match the input impedance of the source to the output impedance of the means for supplying RF energy for the antenna. The tune means may be a variable capacitance electrically connected between one end of the antenna and RF ground. The load means may be a variable capacitance electrically connected between the other end of the antenna coil and RF ground. RF energy may be applied via a tap at a selected location along the coil antenna.
In another aspect, the system includes a dielectric dome or cylinder which defines the source region. Preferably, the coil antenna surrounds the dome for inductively coupling the high frequency electromagnetic energy into the chamber. The article which is fabricated can be located within the source region or dome, within or closely adjacent the volume or the bottom turn of the antenna, or preferably, downstream of the antenna.
Our invention also includes means for supplying gas to the chamber which comprises a gas inlet at the top of the dome, a first ring manifold at the base of the dome source region, and a second ring manifold surrounding at the wafer support electrode, for selectively supplying processing diluent, passivation and other gases to the chamber.
In yet another aspect, an AC power supply and control system capacitively couples AC bias power, typically of the same or similar frequency as the source coil power, to a wafer support cathode, thereby effecting control of the cathode sheath voltage and ion energy, independent of the plasma density control effected by the source radio frequency power. The system provides bias frequency selected to achieve a number of objectives. First, the upper frequency limit is selected to prevent xe2x80x9ccurrent-inducedxe2x80x9d damage (a too high frequency can cause charge-up damage to sensitive devices.) The lower frequency limit is selected in part to preclude xe2x80x9cvoltage-inducedxe2x80x9d damage. Lower frequency bias also yields higher wafer sheath voltages per unit bias power (less heating of substrates) and contributes less to plasma density and thus affords better independent control of ion density and energy. However, a too low bias frequency allows ions to follow the RF component of the wafer sheath electric field, thereby modulating ion energies. The result is a higher peak-to-average energy ratio and wider (double peak) ion energy distribution. Very low bias frequency causes insulator charge-up, inhibiting ion-induced processes during part of the bias frequency period. Conveniently, the preferred frequency ranges for satisfying the above considerations correspond to the source frequency ranges. That is, preferably LF/VHF (low frequency to very high frequency) power within the range 100 KHz to 100 MHz is used. More preferably, LF/HF power within the range 100 KHz to 10 MHz is used. Most preferably, MF (medium frequency) power is used within the range 300 KHz to 3 MHz.
Our invention further includes control means for cyclically pulsing the DC bias voltage between low and high values selected, respectively, to form a passivation coating on a first selected material on the wafer for providing a relatively low etch rate of that material and for selectively etching a second selected material at a relatively high rate and selectivity.
In another aspect, the chamber is evacuated by a first vacuum pump means connected to the chamber proper and a second vacuum pump means connected to the dome for establishing a vertical pressure differential across the dome for establishing a flow of neutrals ions out of the dome, and wherein the voltage at the wafer support electrode is sufficient to overcome the pressure differential so that charged particles flow toward the chamber proper.
Other aspects include a conductive, Faraday shield of different configurations which is interposed between the coil antenna or other coupling means and the chamber to prevent coupling of the electric field component of the high frequency electromagnetic energy into the chamber. Also, a high frequency reflector positioned surrounding the coil or other coupling means concentrates radiation of the high frequency energy into the chamber.
Magnetic enhancement is supplied by peripheral permanent or electromagnet arrangements which apply a controlled static magnetic field parallel to the axis of the antenna, selected from uniform, diverging and magnetic mirror configurations, for controlling the location of and the transport of the plasma downstream relative to the wafer. Also, magnets may be mounted around the source and/or the chamber for applying a multipolar cusp field to the chamber in the vicinity of the wafer for confining the plasma to the wafer region while substantially eliminating the magnetic field across the wafer. In addition, a magnetic shunt may be positioned surrounding the wafer and the wafer support electrode for diverting any magnetic field from the wafer support electrode.
The system construction permits scaling of its size by selecting the frequency of operation, while retaining low mode operation.
In other, process aspects, our invention is embodied in a process for generating a plasma, comprising providing a vacuum chamber having source and process regions; supporting an article on an electrode in the process region; supplying processing gas to the chamber; using a cylindrical coil antenna of more than one coil turn having an electrical length  less than xcex/4 wherein xcex is the wavelength of RF energy applied to the antenna, inductively coupling RF energy into the source region for generating a plasma to fabricate one or more materials on the article; and capacitively coupling RF energy into the chamber via the support electrode for controlling sheath voltage at the support electrode.
The process also encompasses automatically and iteratively tuning the antenna to resonance and loading the input impedance thereof to the impedance of the RF energy supply for the antenna.
In another aspect, our process for generating a plasma comprises providing a vacuum chamber having source and process regions, and having walls, an electrode in the process region and an electrode in the source region; connecting the electrode in the process region, the walls of the chamber and the source electrode electrically, with the process region electrode being the cathode, the walls being the anode and the electrical connection of the source electrode being selected from ground, floating and RF or DC bias; supporting an article on the electrode in the process region; supplying processing gas to the chamber; using a cylindrical coil antenna of one or more coil turns and having an electrical length  less than xcex/4 where xcex is the wavelength of RF energy applied to the antenna, inductively coupling RF energy into the source region for generating a plasma to fabricate one or more materials on the article; and capacitively coupling RF energy into the chamber via the support electrode for controlling sheath voltage at the support electrode.
At least one of the source electrode and the chamber wall in the source region may be or contain silicon and the source electrode may be RF biased, for freeing the silicon into the plasma to enhance the processing.
In another aspect, the antenna power and the bias power delivered to the electrode are controlled for selectively effecting anisotropic, semi-anisotropic and isotropic etching.
The process encompasses etching silicon oxide in the presence of silicon, the use of silicon enhancement, and/or the use of additives such as CO and CO2 for selectivity and etch profile enhancement. The process encompasses cyclically driving the bias voltage to a low value selected to form an etch suppressing layer on the silicon and to a high value to etch the silicon oxide at a high rate relative to the silicon.
The process also comprises sputter deposition of silicon oxide and the process of, first, applying relatively low level RF power to the support electrode for depositing silicon oxide and, second, applying relatively high level RF power to the support electrode for net sputter facet depositing silicon oxide and planarizing the silicon oxide.
Specific process aspects include but are not limited to etching oxide, including etching contact holes in oxide formed over polysilicon (polycrystalline silicon) and etching via holes in oxide formed over aluminum; so-called xe2x80x9clightxe2x80x9d etching of silicon oxide and polysilicon; high rate isotropic and anisotropic oxide etching; etching polysilicon conductors such as gates; photoresist stripping; anisotropic etching of single crystal silicon; anisotropic photoresist etching; low pressure plasma deposition of nitride and oxynitride; high pressure isotropic conformal deposition of oxide, oxynitride and nitride; etching metals, such as aluminum and titanium, and compounds and alloys thereof; and sputter facet deposition, locally and globally, and with planarization.